I found that enabling Vertical Sync via the NVIDIA Control Panel fixed the mouse lag for me. Open NVIDIA Control Panel. Go to Manage 3D Settings. Add Darwinia to the game list, then set the option. I think ATI gfx card owners can do the same thing via the Catalyst control panel.

If that doesn't do it for you, other posts I've seen suggest downgrading the game to the 1.42 OpenGL patch (rather than the 1.5.11 latest patch.) I only experienced the mouse lag once I'd patched the game, so it's a problem that's introduced in the latest patch, rather than a standard 'feature'.You'd probably have to download and install this manually, since Steam doesn't do downgrades (or selective patch installs) and set the game to never update and don't verify the game cache to stop it reapplying the later patch.There's not much functionality difference between 1.42 and 1.5.11, the later one is primarily a port from OpenGL to DirectX graphics.

Change log for 1.5.11 patch (i.e. what you'd lose if you went down to 1.42):- Ported graphical engine from OpenGL to DirectX - Graphical improvements: >> Reflective Water >> Ripple effect on explosions >> Lensing effect on souls- Supports Microsoft XBOX 360 Controller- All language translations updated, we now support English, French, German, Italian and Spanish- Fixes >> Control towers now set their global online status to true when reprogrammed >> AISpawnPoints can now be given a limit to the number of times they spawn